Fix spelling in warning on cHPI (#12095)

Author: Mathieu Scheltienne

mne/_fiff/write.py

@@ -133,9 +133,8 @@ def write_string(fid, kind, data):
     except UnicodeEncodeError:
         str_data = str(data).encode("latin1", errors="xmlcharrefreplace")
     data_size = len(str_data)  # therefore compute size here
-    my_dtype = ">a"  # py2/3 compatible on writing -- don't ask me why
     if data_size > 0:
-        _write(fid, str_data, kind, data_size, FIFF.FIFFT_STRING, my_dtype)
+        _write(fid, str_data, kind, data_size, FIFF.FIFFT_STRING, ">S")
 
 
 def write_name_list(fid, kind, data):

mne/chpi.py

@@ -629,8 +629,8 @@ def _setup_hpi_amplitude_fitting(
         for key in ("sss_info", "max_st"):
             if len(ent["max_info"]["sss_info"]) > 0:
                 warn(
-                    "Fitting cHPI amplutudes after Maxwell filtering may not to work, "
-                    "consider fitting on the original data"
+                    "Fitting cHPI amplitudes after Maxwell filtering may not work, "
+                    "consider fitting on the original data."
                 )
                 break